Which memory card for the 5N?

I had a 1G, but the lock tab broke, so I need a new card. It was a base SanDisk. I want a quality card this time. So what size should I get? The 1G was barely cutting it, but how large should I go? Also, what speed (I don't know if that's the proper term)?
Lastly, is eBay OK for this purchase?